In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as a substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate.
Multilayer cards and boards are typically constructed of a number of layers of copper conductor patterns separated by layers of dielectric plastic. These structures can be produced by laminating techniques.
In order to provide electrical connection between layers, holes referred to as "through-holes" are provided in the circuit boards and cards and such are subsequently plated with an electrically conductive coating such as copper.
However, in the process for producing the holes, invariably some amount of contaminant is formed within and around the edges of the holes.
For instance, when drilling a dielectric substrate such as obtained from an epoxy material, what is commonly referred to as "epoxy smear" due to the drilling results within the hole and around the edges of the hole. Accordingly, prior to electrically plating the holes, the contaminant must be removed to as great an extent as possible. Various treatments have been suggested for removing such contaminant including subjecting the substrate to a gaseous plasma. However, it has been observed that while many gaseous plasma systems are effective for removing the contaminant within the vicinity of the edges of the holes that such are much less effective in removing contaminant from the interior of the holes and particularly within the vicinity of the interplanes of the holes.
The interplanes of the holes represent those locations within the holes where the copper conductor layers exist.
One troublesome problem with plasma etching to remove contaminant from holes is that if the etching process is continued for sufficient time in order to remove substantial contaminant from the interior of the holes, then the outer edges of the holes due to the additional etching time become over-etched. These problems are particularly pronounced when dealing with holes having a relatively high aspect ratio. The aspect ratio is the depth of the hole relative to its diameter.
The above-discussed problem, as well as one particular technique for overcoming the problem, is disclosed in U.S. Pat. No. 4,654,115 to Egitto, et al., disclosure of which is incorporated herein by reference.